Dashcode is your friend
If you have installed the latest Developer Tools, you have probably got Dashcode on your Mac (assuming you have a Mac). Dashcode was updated for the iPhone SDK, and is now a very good tool for building web apps that use 'proper' iPhone user interfaces.
If you're not using a Mac, or you don't have access to the latest versions of the Developer Tools, check out
IUI, which provides some very nifty libraries for iPhone web apps.
Dashcode contains a tutorial, but IUI will need you to play around with it a bit and do some Googling in order to understand it properly. If you already know HTML and PHP, though, they should both be fairly straightforward to use.
